1. Field of the Invention
The present invention relates to methods of plating metals onto other metals. More particularly it concerns electroless plating methods, i.e., those methods in which no external electric current is applied to initiate or sustain the plating reaction. The present invention relates to methods of plating metals onto more electropositive porous metal surfaces by chemical reducing of cations of the metal to be plated by atoms of the metal being plated. This method requires no chemical reducing agent in addition to the porous metal surface itself.
2. Description of the Related Art
Electroless methods of depositing metals on other metals are well known. They are disclosed for example in U.S. Pat. Nos. 2,532,283; 2,762,723; 2,935,425; 2,999,770; 3,338,741; 3,202,529; 3,121,644; 3,264,199; and 3,148,072. These methods are self-sustaining and require no application of the external current needed in other electroplating techniques. The known methods normally involve immersing the metal substrate to be plated in a solution containing cations of the metal to be deposited. The methods may also employ complexing agents to keep the cations in solution until plated, ingredients to adjust pH, and a means for heating the solution to an optimum temperature for the particular reactions being employed. The plating reaction is normally catalyzed at first by the metal substrate being plated and, at a later point in the process, by the deposited metal itself.
To sustain the plating process after depositing a few atomic thicknesses of the plating metal, the known electroless plating processes require a chemical reducing agent to reduce the cations of the metal to be deposited. For example, electroless nickel plating processes normally employ sodium hypophosphite as the chemical reducing agent for the nickel cations. However, the hypophosphite process is inconvenient for a number of reasons. Much of the hypophosphite is wasted because it reacts with water, rather than the metal cations. This reaction results in the formation of gaseous hydrogen, which causes unwanted bubbling in the plating solution. Such bubbling can sometimes become uncontrollable. In the process of depositing nickel on iron using a hypophosphite reducing agent, solutions must be formulated in large, dilute volumes, because the solubilities of nickel and iron phosphites are very low. Even when large volumes are used some nickel and iron phosphite residues form, and the solution must be discarded after each use. This, in itself, is a problem, because simple disposal of phosphorous-containing solutions into public sewer systems is now prohibited by environmental regulations, and processes for reclaiming the phosphite are difficult at best. Also, the utilization of chemical reducing agents inevitably results in a metal deposit that contains some of the elements of the reducing agent in addition to the plating metal itself. In some applications, this is very undesirable and prevents practical utilization of the electroless plating process.
Therefore, it would be desirable if a sustainable electroless plating processes could be carried out without the addition of any external reducing agent. When the metal to be deposited is more electronegative (has a lower oxidation potential) than the metal substrate on which it is to be deposited, the metal substrate itself can serve as the reducing agent for the cations of the plating metal. However, it has been thought that such a process could not be carried out to any useful extent, because after a few atomic thicknesses of the plating metal are deposited, the rate of diffusion of metal substrate ions out into the solution becomes essentially zero, thus effectively cutting off the source of reducing agent for the metal cations. This is described, for example, in, M. Lelental, "Catalysis in Nickel Electroless Plating" J. Electrochem. Soc., 122(4), pp. 486-90 (April 1975). Lelental has indicated that for all practical purposes the maximum thickness of a layer of a plating metal that could be deposited on a more electropositive metal substrate by ion exchange with the substrate beneath the layers already plated is 7.5.times. 10.sup.-7 cm., because at that point the reaction rate would drop to essentially zero. Accordingly, it has been assumed that such a process would not provide a useful method of electroless plating, because generally a plating thickness of at least 2.5.times.10.sup.-6 cm. is necessary to provide a deposit sufficient to cover the underlying surface and more often a plating thickness of about 2.5.times.10.sup.-4 cm. is desired.